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Isotopic composition of gadolinium, samarium and europium in the Abee meteorite Loveless, Arthur John


The writer has measured the isotopic composition of gadolinium, europium and samarium in the Abee meteorite and in two terrestrial ores. Gd, Eu and Sm have large thermal neutron capture cross sections; they may therefore be used to reveal differences in the irradiation histories of samples containing trace amounts of these elements. It is widely believed that the contracting protosun passed through a phase of high energy particle radiation during the early history of the solar system. The interaction of these particles with the material of the solar nebula may have produced a large thermal neutron flux. The Abee enstatite chondrite is representative of the most highly reduced class of chondritic meteorites. The chondrites are stones which are thought to be very primitive material of the solar system. Mason, Miyashiro and others have proposed that variations in the oxidation state of chondrites result from their formation in different regions of the nebula: the enstatite chondrites represent material which was derived from the hot inner region of the solar nebula, while the highly oxidized carbonaceous chondrites had their origin in the cooler, outer region. On the basis of this theory, the earth was probably derived from an intermediate region of the nebula. Isotopic analyses were performed on microgram quantities of Gd, Eu and Sm. A precision of 0.02-0.2% (at the 95% confidence level) was achieved for all isotopic ratios of interest. Prior to 1970 the best published isotopic analyses of these elements had a precision of 1-2%. The higher precision reported here was made possible through improvements in the mass spectrometer ion optics and the use of digital recording of mass spectra. This work is comparable to recent meteorite analyses on Gd by Eugster, Tera, Burnett and Wasserburg at the California Institute of Technology, although their lunar analyses were of superior quality and had a precision of 0.01%. No significant Gd, Eu or Sm isotopic anomalies were observed for the Abee and terrestrial samples. The Gd¹⁵⁷/Gd¹⁵⁸ ratio, which is the most sensitive to neutron irradiation, was identical for the three samples studied within 0.1%. This places an upper limit of 3 x 10¹⁵ neutrons/cm² for the differential irradiation of the source material from which the earth and the Abee meteorite were derived. A similar conclusion was reported by the Caltech group. The absence of any anomalies may be attributed to uniform irradiation and dilution of the source material, efficient shielding inside large planetary bodies during the irradiation phase, the absence of an intense irradiation phase, or a common spatial origin within the solar nebula for chondritic and terrestrial matter.

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